Answer to Question #4173 Submitted to "Ask the Experts"

Let me first say that I am glad that your son had no complications from his initial injury. It should be noted that all medical procedures have some risk, whether they involve flu vaccinations, x-ray exams, or surgery. The choices you made were based on the information and recommendations of medical staff, who have also to weigh their observations of patient symptoms to possible outcomes, both good and bad. The recommendations of the nurse and the emergency room physician were made in the best interest of your son.

From your comments, one can conclude that the medical staff may not be completely knowledgeable about radiation doses. This is not an uncommon situation. Radiation exposures vary from machine to machine and from exam to exam and, unless the physicians specialize in radiology or a related medical field, they look upon CT and other exams as diagnostic tools without delving into more detailed information. There are also different ways of expressing doses, which further adds to the confusion. We could indicate the dose as the amount of radiation that exposes the body, to specific organs irradiated, or to an overall risk estimate to the individual. Generally, we try to define a typical exposure risk to the entire patient, called the effective dose, as organs have different chances of developing cancers based on the radiation received; for example, some organs or tissues are more "radiation resistant" than others.

With regard to the Brenner, et. al. (2001) paper you mention, a number of points need to be made clear. First, the study was not based on the occurrence of cancers in real patients. His analysis is based on studies of dose distributions in patients and estimated cancer risks extrapolated from the doses to the various organs irradiated.

Second, the perspective risks may be overstated based on what is known about the natural cancer risks in children and adults. The following is from the conclusion:

"Although the absolute estimated risks that we have projected are quite high, the percentage increase in the cancer mortality rate over the natural background rate is very low. For example, of the approximately 600,000 children less than 15 years old who are estimated to undergo CT each year in the United States, approximately 140,000 will ultimately die of cancer. Thus, the estimated projected 500 CT-related deaths represents a small (approximately 0.35%) percentage increase over this background. This small estimated relative risk suggests that detection of an increased risk in an epidemiologic study would not be easy, although a recent case-control study [35] on the association between pediatric radiologic examination and childhood leukemia did show a significant elevated risk (linearly related to the number of examinations) compared with controls in children who received two or more diagnostic examinations (odds ratio, 1.6; confidence interval, 1.1-2.3)."

Back to your original posting, I will try to answer all of your questions as they are presented:

1. To the best of my knowledge, there have been no studies that confirm the risk estimates for CT scanning. Nevertheless, it should be noted that studies do show that radiation exposure does increase the risk of cancer in children, as noted in a paragraph above. 
    
2. When we say that there is an increased risk of cancer, we mean that the radiation exposure adds to the normal cancer risk. Currently, we might estimate that 25% of the population will develop cancer, so that in a population of 10,000 individuals, 2,500 will develop cancer. If each and every individual receives 10 rem (0.1Sv) of radiation, then the risk of cancer increases by 0.05%. This will be mean we might expect to see 2,505 cancers in the population. Two things need to be kept in mind. First, these are only estimates. It is possible that the incidence of cancer in this population may range from 2,450 to 2,550 without any addition radiation exposure. Second, we cannot say which members of the irradiated population developed cancer from the radiation. If fact, the odds are that the majority, approximately 75%, will not develop cancer. Having said this, one can say that some individuals who would not have gotten cancer will. Again, it is impossible to identify which individuals in this population will be affected. For those who do develop cancer in this group, the incidences of cancer will statistically appear earlier than if irradiation had not occurred.
    
3. Again, to the best of my knowledge, there have been no specific studies of children who had head CT exams. When evaluating cancer risks, various organs are assigned tissue weighting factors. For example, if the thyroid is irradiated, the cancer risk would be determined by multiplying the exposure by the tissue weighting factor of 0.05. If the whole body is irradiated, the sum of all the tissue weighting factors would total 1.0. Generally, the brain is not specifically identified. However, in the case of a head CT scan, the brain is specifically exposed, so it is assigned a tissue weighting factor of 0.025. Thus, if the radiation dose was to the brain is 2 millisievert (mSv) or 200 mrem, the specific risk to the brain is 5.0 mrem. As I noted above, it is impossible to determine when, or if, cancer will develop to a specific individual in a population where all have received a head CT scan. 
    
4. There is a body of evidence that radiation damage is repaired by the body. Much of what we know about radiation risks is based on high doses received in short periods of time. At low doses and dose rates, there are no demonstrated effects at cumulative doses of 0.1 Sv or 10 rem according to the Health Physics Society document. I am not aware of any studies that show that breast-feeding or any other nutritional program improves the body repair of radiation effects. 
    
5. The actual doses that your son received would have to be determined from measurements made on that machine with those settings. Generally, there is little dose information on doses of CT scans of children for various scanners. In one paper the average effective dose is given as 7.6 mSv or 760 mrem, with a range of 3.5 to 14.5 mSv (Huda et al. 2001). A Posterior/Anterior (P/A) adult chest x ray is about 2 mrem. As I noted earlier, there are many different ways to state a "dose." 
    
6. Our Web site and the Health Physics Society's Position Paper "Radiation Risk in Perspective" say that any risk below 0.1 Sv or 10 rem cannot be demonstrated. If there is an increased risk, we cannot observe it. As I noted above, we try to estimate the risk on the effective dose which incorporates risk to individual organs as the best way to estimate risks as it incorporates the tissue weighting factor or organ risk. 
    
7. Personally, I would not withhold any medical exam from my child if it is recommended by a physician. Radiation is considered a weak carcinogen. The benefits of medical and dental x rays far outweigh the risks. 
    
8. Physicians will continue to recommend CT scans when it is in the best interest of the patient. The Brenner, et. al. study did make the medical community aware that radiation doses are higher than necessary to obtain the information needed according to the US Food and Drug Administration. Consequently, steps have been taken to minimize these doses (Donnelly et al. 2001).

John Jacobus, MS
Certified Health Physicist

References

• Brenner D, Elliston C, Hall E, Berdon W. Estimated risks of radiation-induced fatal cancer from pediatric CT. Am J Roentgenol 176(2): 289-96; 2001.
   
• Donnelly LF, Emery KH, Brody AS, Laor T, Gylys-Morin VM, Anton CG, Thomas SR, Frush DP. Minimizing radiation doses for pediatric body applications of single-detector helical CT: Strategies at a large children's hospital. Am J Roentgenol 176(2): 303-306; 2001.  
• Huda W, Chamberlain CC, Rosenbaum AE, Garrisis W. Radiation doses to infants and adults undergoing head CT examinations. Med Phys 28(3): 393-399; 2001.